A. Upstairs
in a small mall/quad in Newport's Pier Plaza. (See the Pier
Plaza page for more details.)

Q.
When did we see it on the show?

A. Throughout
the final 4th season, in virtually every episode, until it (and much if
Newport) was shut down by the earthquake. It was a convenient place for
the producers to stage conversations with Seth.

We first see
the comic book store in the first episode of Season 4 ("The Avengers"),
in a scene where Seth (working behind the counter) brings a young girl
to tears, because she has dared to ask "Is there a comic book based
on The X-Men movie?" His mom (Kirsten) shows up with a care package
that she wants Seth to bring to the grieving Ryan (who was hiding out in
a fight club).

Later, in the
same episode, the Cohens hold an intervention, of sorts, there for Ryan
(who has flipped out following Marissa's death), where Seth shows Ryan
slides of a comic book he has created about how Ryan has helped the Cohen
family.

It's where Taylor
later tells Seth that she's falling for Ryan, and tries to recruit him
to help her effort (in Episode 5, of Season 4, "The Sleeping Beauty").

It's where (oddly
enough) Ryan and Taylor have their first date - watching anime movies on
a flat-screen TV and eating popcorn - , which doesn't work out well (in
Episode 6 of Season 4, "The Summer Bummer").

(The set is
loosely based on a real building/quad on the north side of Hermosa Beach's
real Pier Plaza.)

Q.
Where can I find it in real life?

A. You can't.
This is one of the more disappointing locations in The OC.

The comic book
store (and the two-story building where it appears to be) does not exist
in the real world. It is simply part of an elaborate Pier Plaza set (which
also contained Ryan's Mexican restaurant)
inside a sound stage at Manhattan Beach Studios.

The set was
based loosely on the Pier Avenue Promenade in Hermosa
Beach, and a particular shopping center there (Loreto Plaza),
but other than the establishing shot (which is a real shot of that Hermosa
Beach promenade) nothing else about the comic book shop was filmed there.